1. Field of the Invention
The present invention generally relates to touch screens, and more particularly, to an improved controller for resistive touch screens.
2. Description of the Related Art
Pressure sensitive touch screens have become more prevalent in our society in recent years and are used in many diverse applications such as personal digital assistants, portable instruments, point-of-sale terminals, pagers, and various other touch screen monitors. Touch screens can be activated by many different types of contacts, including by finger, pen, and stylus. The user touches different areas of the touch screen to activate different functions. In addition, the user may use a pen or similar tool to write directly on the touch screen.
The two most popular touch technologies are analog capacitive and resistive touch screens. A resistive touch screen works by applying a voltage across a resistor network and measuring the change in resistance at a given point on the network where a screen is touched by an external source. The two most popular resistive architectures use 4-wire (400) or 5-wire (410) configurations as shown in FIG. 4. A typical resistive touch screen includes a resistive layer on an insulating material such as a glass panel. A hard-coated, conductive topsheet is overlaid on top of the resistive layer. The topsheet is separated by a layer of small insulating spacers from the resistive layer. When the surface of the conductive topsheet is touched by an external implement, for example, by pen or similar tool, or by a user""s finger, the conductive coating on the topsheet is pushed against the resistive layer making electrical contact. As a result, voltages are produced that provide the analog equivalent corresponding to the position touched.
A touch screen controller, such as, for example, the ADS7845 touch screen controller from Texas Instruments Tucson Corporation, drives the touch screen. The touch screen controller has a terminal, such as a pen interrupt pin, dedicated to producing a signal when the screen is touched. Initially, the pen interrupt signal will be at a high voltage. The touch screen controller monitors the touch screen and pulls the pen interrupt signal to a low voltage when the screen is touched. The pen interrupt signal may then be output to a microprocessor. The microprocessor will then signal an analog-to-digital converter within the touch screen controller to initiate conversion of the analog voltage of the touch screen into a digital signal representing the x,y coordinate position of the touch. Since the pen interrupt pin of the touch screen controller chip is dedicated to the pen interrupt signal, the pin cannot be used for other purposes.
The pen interrupt signal is a digital signal that is pulled low when a current flow is detected from the depression of the touch screen. Further, the pen interrupt signal has slow switching edges and contributes to a slow reaction time when the screen is touched. This slow reaction time is also caused by the microprocessor setting the pen interrupt diode into a reverse bias state. Otherwise, if the pen interrupt diode is forward biased during a conversion, the additional current will cause the input data to be inaccurate.
Thus a touch screen controller is needed that will have a faster response when the screen is touched. In addition, minimizing the power consumption of the touch screen controller while waiting for touch screen input will significantly aid low power applications such as Personal Digital Assistants. Further, with the increased demand in touch screen controllers for more flexibility and additional input/output terminals, eliminating the need for a dedicated pen interrupt terminal can provide significant advantages to system designers.
A touch screen controller in accordance with the present invention addresses many of the shortcomings of the prior art. In accordance with one aspect of the present invention, the touch screen controller is configured to detect a screen touch while the touch screen controller is powered down, and the detection signal is output on a data output terminal of the touch screen controller. In accordance with an exemplary embodiment, by monitoring the analog signal from the touch screen through a static comparator, a digital signal may be generated at no power consumption. This digital signal may be output on the data output pin of the touch screen controller while the controller is in a power down mode. Thus, an additional I/O pin is not needed by the controller to monitor whether a screen touch occurs.